ECONNREFUSED from inside a Toolbox

Hi all. On my new Silverblue setup, I’m trying to use an npm package that wants to drive Ungoogled Chromium from inside a Toolbox. On the base system, this works fine:

CHROME_PATH="/var/lib/flatpak/exports/bin/com.github.Eloston.UngoogledChromium" npx web-ext run --target chromium

However, when I try to run the same command from inside my Toolbox, I see the following:

Error: connect ECONNREFUSED
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1159:16)


How do I set up the Toolbox to be able to communicate over that port?